[Ask] Fenix AA/14500 effective working voltage

Hi all,

I plan to UV convert a flashlight. The problem is most UV LED have 3,5V - 3,7V vf. Converting normal boost driver FL limits me to single AA and AAA FL. While converting normal buck driver FL will get me very short regulated runtime. I've been looking for the widest input voltage single battery flashlights (buck - boost driver). This way, I can make the most out of a single Li-Ion battery down to 2,8V before the battery protection kicks in. By the nature of UV LED, I don't need those powerful high current FL. Any FL with output less than 600 lumens is better. UV LED can't stand the heat as much as white LED (100C max).

I ended up with Fenix as the only manufacturer who probably has it. The RC05, LD09, and LD12 seems the best candidates for their low output, wide input voltage. But I don't have them to confirm.

The other AA/14500 FL always have different outputs - higher when using 14500. Unlike Fenix, it's AA/14500 FL maintain low - med - high brightness in regulation no matter what battery you put in. The only difference are longer runtime and brighter turbo (higher high) with 14500. Could it be that Fenix has buck/boost 0,8V - 4,35V super wide input voltage?

Any inputs or thoughts? Especially from Fenix RC05, LD09, and LD12 owners?

Thanks,

- Clemence

I don’t know about Fenix, but the Olight S1A is a dual AA/14500 flashlight. It also has a higher output with 14500 than with AA. But it has a “gap” that I found when trying to test with LiFePO4. You can just access Turbo, then it immediately kicks down to a lower level. In other words, the driver wasn’t made with the “middle” voltage of LiFePO4 in mind.

If your Vf is 3.5V to 3.7V range, you should possibly get a boost driver made specifically for 2xAA and/or LiFePO4 and/or CR123A. Those drivers can usually handle the 4.2V max charge of Li-Ion, but also won’t step down output when using a ~3V input, since that’s what they’re made for.

Yes, boost drivers will amost always have flatter runtime curve which I like. Never heard any driver like that (2 x AA boost). I actually interested in the 2,7-5,5 16mm driver HKJ reviewed years ago. The last time I saw it was in Hank’s store but he doesn’t sell it anymore.

Usually, when you’re looking at AA boost drivers, you end up finding 1 - 2 AA only (top voltage ~3V for input). But, if you look at complete flashlights, you can find drivers with a higher range of voltages.

The Zanflare F2 is a 2xAA light which uses a boost driver. I have a F2S, the newer version, which I’m past due for uploading a review for. I need to get it up soon. Anyway, I’ve found that it can take as low as single AA NiMH voltage input, but also up to 4.2V Li-Ion (only tested briefly). Since it’s made for ~3V input (2xAA alkaline) it works well with LiFePO4 as well. I wouldn’t recommend it for your project, because it has no LVP. But, it is an example.

Also, there are many CR123A lights out there. CR123A is a ~3V lithium primary cell. So, the drivers for those are all going to be boost drivers. And some of them can take 4.2V Li-Ion 16340 as well. I had one before that needed something like ~1.7V to turn on, but was good all the way up through 4.2V Li-Ion.

I don’t know if there are drivers made specifically for LiFePO4. Probably not. But, if you find one, it would work for you too. You just need a boost driver that can take a high input voltage relative to the output. Any of these choices will do. You’re going to have a better chance by buying a complete flashlight.

Correct, using any single CR123 high output FL will gets the job done but most of them will output too much when used with 4,2 Li-Ion batt. Even in US where CR123 is widely accepted it’s still expensive for us budgeteer, let alone in my country.
A buck boost driver is needed here. First, bucking down 4,2V to 3,7 and then boosting up when the batt gets lower than 3,7 all the way to 2,8V.
With dual 2S Li-Ion the problem solved cheaply (bucking only) but I like the small single Li-Ion form factor.

I tested my predator pro (XPL) which said to has buck-boost driver but it is not. The regulation fall off rapidly once the voltage hits 3V (using external controlled DC power source, full power output).

How could the output be too high with a CR123A flashlight? If you’re using a 16340 in it, there shouldn’t be much potential to overdrive it, because of the small cell size. And, even if it might be too high output on high mode, you could use a lower mode instead. If nothing else, the higher Vf of the emitters you’re planning to use should keep the output low enough anyway, I’d think.

I don’t know. I’m just trying to help. Maybe someone else will chime in with a better solution.

Don’t get me wrong Dave, really appreciate the thoughts.
I don’t know but in my thinking, if a 3V designed boost driver will always gives out higher voltage at certain set current.
For example:

3V batt + boost driver driving 3,7V LED at 500mA

What will happen if the batt voltage changed to 4,2V? < this is truly a question :blush:

- Clemence

What do you think about this? To me it’s definitely a buck boost FL
Having 3V from two Eneloop is much cheaper than CR123. I think a single battery FL is not a good choice for my build. The next problem would be to mod it. Fenix loves glue so much

https://www.fenixlighting.com/product/e25ue-fenix-flashlight/

Edited: wrong FL link